Photodetectors are widely used to detect the intensity of light impinging upon the photodetector by converting the flux of light photons into an electronic current, which is then measured by conventional electronic means. Two primary operational parameters of a photodetector are its sensitivity to light within the desired spectral band, that is, the size of the electrical current output by the photodetector relative to the number of photons incident upon the photodetector, and the noise output by the photodetector or its associated circuitry. A high light sensitivity is desired, but an adequately high signal-to-noise ratio must be maintained or the random noise signal will mask the light-derived signal.
Many types of photodetectors are available for the light spectrum ranging from the infrared to the ultraviolet. In particular, semiconductor photodiodes are readily available and moderately inexpensive and have found widespread use. However, their sensitivity is insufficient for some very advanced applications in the long wavelength region in which the light has a wavelength longer than about 1 .mu.m, that is, an energy less than 1.24 eV. This range includes the 1.3 and 1.55 .mu.m bands that are being used for optical fiber communication. It is expected that fairly inexpensive and rugged photodiodes made of III-V semiconductors will be operationally used in most applications. However, more demanding applications require a more sophisticated detector with a higher signal-to-noise ratio and a high bandwidth with low photon fluxes. One effective photodetector in the long-wavelength region is an intensified photodiode (IPD) tube based on a transferred electron (TE) photocathode. This photodetector is often referred to as a TE-IPD. In very general terms, long-wavelength photons incident on the photocathode cause the cathode to emit electrons. An electron detector then measures the number of electrons in the flux emanating from the photocathode.
Bell in U.S. Pat. No. 3,958,143 discloses a highly effective photocathode mechanism in this wavelength band. His structure involves a transferred electron device, for instance, including a p-type InGaAsP active layer sandwiched between a p-type InP substrate and a lightly doped InP surface layer. As is explained by Bell, when the resulting semiconducting structure is biased with the surface layer positive, electrons are injected into the conduction band of the InP surface layer. The injected electrons are promoted to higher-mass conduction valleys in the InP surface layer, and at these higher energies a significant fraction of the electrons will be transported with a minimal loss of energy through the Schottky barrier created at the semiconductor/metal interface between the InP surface layer and the electrode. Therefore, cathodes which have been activated with a surface layer of Cs.sub.x O.sub.y in order to lower the energy of the electron vacuum level to below that of the high-mass valleys can demonstrate very high photoemission yields.
Costello et al. in U.S. Pat. No. 5,047,821 disclose a similar device structure and also provide some details of a gridded electrode structure which more effectively biases the thin metallization layer of the Schottky barrier.
Aebi et al. in U.S. Pat. No. 5,326,978 and La Rue et al. in U.S. Pat. No. 5,374,826 disclose a focussed electron beam (FEB) tube structure usable with the transferred electron (TE) photocathode of Costello et al. These patents, as well as describing the embodiment that uses the photocathode, also describe an embodiment that instead uses a multi-channel plate, which is a planar photomultiplier tube, but that embodiment is not relevant to the present invention. In the structure of these patents that use the photocathode, a fairly large photocathode is positioned at one end of a tube and is biased negatively with respect to an electron detector at the other end. The photocathode converts long-wavelength photons to electrons with high efficiency. A set of annular electrodes are disposed about the axis between the photocathode and the electron detector in order to focus the electrons on the detector. This technology has also been described in the technical literature by Costello et al. in "Transferred electron photocathode with greater than 5% quantum efficiency beyond 1 micron," SPIE Proceedings, vol. 1449, 1991, pp. 40-50, by La Rue et al. in "High quantum efficiency photomultiplier with fast time response," SPIE Proceedings, vol. 2022, 1993, pp. 64-73, and by Costello et al. in "Transferred electron photocathode with greater than 20% quantum efficiency beyond 1 micron", SPIE Proceedings, vol. 2550, 1995, pp. 177-187.
Although such an FEB-TE tube, as described in the prior art, provides very high performance, it suffers several disadvantages, many of which are related, we have found, to the custom fabrication of its photocathode. The transferred-electron photocathode of Bell and Costello is based upon a III-V semiconductor heterostructure grown on InP substrates, which at the present time are typically available in a size having a 2-inch (50 mm) diameter. A conventional fabrication process will now be described for converting the deposited InP substrates into photocathode cells.
In a first step, the 2-inch InP wafer is grit blasted to form three 0.855-inch (18.6 mm) diameter circular cuts that are separated from each other. On average, at the end of processing and packaging, only two of the cuts form operable devices. That is, on average this process forms only two usable cuts from the 2-inch wafer. The grit blasting step typically requires two hours.
In a second step, each of the individual cuts is mechanically masked on its backside and an ohmic contact layer is e-beam deposited through the mask. A mechanical mask is a free-standing metallic sheet that has the desired pattern machined through it, and then the deposition beam is directed through the mechanical mask towards the substrate. The mechanical mask shadows the portions of the substrate not to be deposited upon. This masked deposition step typically requires 3 hours.
In a third step, the backside of each cut is again mechanically masked for the deposition of an anti-reflection coating by plasma-enhanced chemical vapor deposition (PECVD). This step typically requires 1 hour.
In a fourth step, the front of the cuts are deposited with a contact metal. This step typically requires 3 hours.
In a fifth step, the individual cuts have their frontsides photolithographically masked for a step of etching the contact grid pattern into the contact metal over the desired active region of the cathode. This step typically requires 4 hours.
These steps and their required times are summarized in TABLE 1.
TABLE 1 ______________________________________ 1 Grit blast 2 hours 2 Mask back and deposit contact 3 hours 3 Mask back and deposit AR coating 1 hour 4 Deposit contact metal on front 3 hours 5 Mask and etch contact grid pattern 4 hours TOTAL 13 hours ______________________________________
These steps complete the fabrication of the individual photocathode cells, which are then manually assembled into the tubes. The table shows, however, that the process for completing fabrication on average of two photocathodes from a 2-inch wafer requires about 6.5 hours per photocathode. The prior-art fabrication is thus labor intensive and causes the resulting photodetector tubes to be expensive.
Wafers of InP grown with the desired TE heterostructure are expensive. The above prior-art process produces a typical yield of only two photocathodes per 2-inch wafer. Furthermore, the performance of TE photodetectors is limited by dark-current noise unless the photocathode is cooled. Extensive cooling is both expensive and cumbersome, but dark current can alternatively be reduced by reducing the size of the photocathode. This reduction can be shown by the fact that the noise effective power that is limited by dark current NEP.sub.dc can be expressed analytically as ##EQU1## where hv is the photon energy, .eta. is the quantum efficiency, f is the excess noise factor, J.sub.d is the dark current per unit area, A is the area of the detector, .DELTA.f is the bandwidth in hertz, and e is the electronic charge. Hence NEP.sub.dc is proportional to the square root of the area of the photocathode and can be reduced by reducing the area. Lenses are available at the desired wavelengths so that size reduction does not degrade overall performance for most applications. Although both considerations suggest reducing the size of the photocathodes, the prior art steps of mechanical masking and even handling the cuts for steps such as photolithography become difficult when the cathode size is reduced to much below 0.8 inch (2 cm).
Finally, the prior-art process involved in fabricating the photocathode of Costello involves many manual steps, which are prone to error and difficult to incorporate into a production line.